1. Field of Invention
The present invention relates to a method of constructing towers such as tall concrete chimneys or cooling towers and to an apparatus such as a derrick for use in such a method.
2. Description of the Prior Art
In the construction of tall concrete structures such as chimneys, cooling towers, and the like it has long been a practice to construct a derrick comprising one or more work decks from which the casting of concrete of the tower is done. Stirrups are conventionally cast into each level of the concrete tower as anchor points. As the concrete hardens, cables are attached to these stirrups and the derrick is supported by these cables on the stirrups. As the tower is built higher, the derrick is raised to be supported by stirrups at higher levels. The stirrups being used at any given time are those mounted at a level below that which is being poured.
Derricks of this type have been constructed of wood for more than 30 years. An improved alloy structure for such derrick was disclosed in U.S. Pat. No. 4,122,646 issued to Sapp in 1978.
Typically, in the use of such derricks, a work day begins with the derrick supported by cables anchored to a level of the tower poured three days before. There may be about 12 such cables, running from anchor points on the tower, under the derrick where they are attached at 24 points to the derrick, and where they form a basket-like cradle for the derrick, to 24 anchor points on the opposite side of the tower. About 12 cables are connected by chain blocks to an angle ring braced against the outside forms of the tower. A large crew of men then jacks up the derrick by tightening the chain blocks on each of the 12 cables, taking care to tighten each block evenly with the others so as to keep the derrick supported in a level attitude. When workers tighten these block unevenly, they generate large internal stresses in the derrick, which stresses are particularly hard on a wooden derrick, and which may deform the angle ring.
The outer forms which were used to cast the previous day's pour are then attached to chain-hoists mounted on upper outriggers on the derrick. The forms are broken loose and hoisted by the chain-hoists to the next level and secured in place. Reinforcing steel, including anchoring stirrups, is placed inside the outer forms. A work scaffold, comprised of a plurality of parallel beams, is then raised by chain hoists. Inner forms are broken loose and hoisted by chain hoists, then placed inside the reinforcing steel. Concrete is then poured from the derrick, between the outer and inner forms, encapsulating the reinforcing steel within. The concrete hardens overnight and the procedure is repeated again next day at a new level.